WHERE THE IMAGINAL APPEARS REAL - A POSITRON-EMISSION-TOMOGRAPHY STUDY OF AUDITORY HALLUCINATIONS

An auditory hallucination shares with imaginal hearing the property of being self-generated and with real hearing the experience of the stim ulus being an external one. To investigate where in the brain an audit ory event is ''tagged'' as originating from the external world, we use d positron emission tomography to identify neural sites activated by b oth real hearing and hallucinations but not by imaginal hearing, Regio nal cerebral blood flow was measured during hearing, imagining, and ha llucinating in eight healthy, highly hypnotizable male subjects prescr eened for their ability to hallucinate under hypnosis (hallucinators). Control subjects were six highly hypnotizable male volunteers who lac ked the ability to hallucinate under hypnosis (nonhallucinators). A re gion in the right anterior cingulate (Brodmann area 32) was activated in the group of hallucinators when they heard an auditory stimulus and when they hallucinated hearing it but not when they merely imagined h earing it, The same experimental conditions did not yield this activat ion in the group of nonhallucinators. Inappropriate activation of the right anterior cingulate may lead self-generated thoughts to be experi enced as external, producing spontaneous auditory hallucinations.